1. Field of the Invention
The present invention relates to a capacitor type acceleration sensor based on the electrostatic servo system for sensing an acceleration by making use of a variation of capacitance of a capacitor.
2. Discussion of the Related Art
A capacitor type acceleration sensor based on the electrostatic servo system is a typical example of the acceleration sensor. The capacitor type acceleration sensor is fabricated by a silicon microfabrication technique. The capacitor type acceleration sensor has a broad dynamic range and a high sensitivity. Because of advantageous features, the acceleration sensor has the attraction of designers in this field.
FIG. 4 is a block diagram showing the circuit arrangement of a conventional capacitor type acceleration sensor based on the electrostatic servo system. The mechanical construction of an acceleration sensing element 1 used in the acceleration sensor of FIG. 4, is schematically illustrated in FIG. 7. As shown, a movable electrode 4 is disposed at the mid position (neutral position) between fixed electrodes 2 and 3. The movable electrode 4 is supported at both ends thereof by a couple of cantilevers 5 in such a way when receiving an acceleration, the movable electrode 4 is displaced in the Y directions. Returning to FIG. 4, a variable capacitor C1 is constructed with the fixed electrode 2 and the movable electrode 4, and another variable capacitor C2 is constructed with the fixed electrode 3 and the movable electrode 4. The variable capacitors C1 and C2 are fabricated by the silicon microfabrication technique.
A sinusoidal wave signal of an amplitude Vp and at an angular frequency .omega., generated by an oscillator 6, is applied to the variable capacitor C2 of the acceleration sensing element 1. A phase invertor 9 is inserted between the oscillator 6 and the variable capacitor C1. The sinusoidal wave signal, generated by the oscillator 6, is phase shifted by 180.degree. by the phase invertor 9, and applied to the variable capacitor C1. The amplitude and the angular frequency of the sinusoidal wave signal inverted and applied to the variable capacitor C1 are also Vp and .omega.. The acceleration sensing element 1 is connected at the output terminal to a capacitance-difference detector circuit 10, which functions as a capacitance-difference detecting means for detecting a capacitance difference between the variable capacitors C1 and C2, and outputs in the form of power signal. The capacitance-difference detector circuit 10 includes an operational amplifier A1. The output terminal of the acceleration sensing element 1 is connected to the inverting input terminal of the operational amplifier A1. The noninverting input terminal of the operational amplifier A1 is earthed. A feedback resistor Rf is inserted between the inverting input terminal and the output terminal of the operational amplifier A1.
As shown in FIG. 4, a synchronous detector circuit 12 is inserted between the oscillator 6 and the capacitance-difference detector circuit 10. An electrostatic power generation/feedback means 15 is inserted between the output terminal of the capacitance-difference detector circuit 10 and the acceleration sensing element 1. The electrostatic power generation/feedback means 15 applies an electrostatic power to the fixed- and movable-electrode pairs 2 and 4, and 3 and 4. The electrostatic power sets the movable electrode 4 at a preset reference position (the neutral position) in accordance with the output signal of the capacitance-difference detector circuit 10. The electrostatic power generation/feedback means 15 includes an integrator 11, a comparator 13, and a feedback ratio setting circuit 14. The integrator 11 detects an output voltage of the capacitance-difference detector circuit 10, integrates it, and outputs the result of the integration to the comparator 13.
The comparator 13 compares the output voltage signal of the capacitance-difference detector circuit 10 with a reference voltage. More specifically, the output voltage signal of the capacitance-difference detector circuit 10 is integrated by the integrator 11. The output voltage signal of the integrator 11 and a reference voltage are compared by the comparator 13. The reference voltage is equal to the integrated value (offset) of the output voltage of the capacitance-difference detector circuit 10 when the acceleration is zero. That is, it is zero. The feedback ratio setting circuit 14 applies the output signal of the comparator 13 as a servo feedback voltage to the fixed- and movable-electrode pairs 2 and 4, and 3 and 4. Since the feedback voltage is set on the basis of a state that the acceleration is zero, the movable electrode 4 of the acceleration sensing element 1 is controlled so as to be at the position thereof when the acceleration is zero. Accordingly, the output voltage of the capacitor type acceleration sensor when the acceleration is zero is the feedback voltage itself.
As recalled, the sinusoidal wave signals which are respectively applied to the fixed electrodes 2 and 3 are out of phase, but are equal in the amplitude Vp and the angular frequency .omega.. An amplitude V.sub.A of the output voltage V.sub.OUT1 of the capacitance-difference detector circuit 10, which receives the output signal from the acceleration sensing element 1 receiving such sinusoidal wave signals, is mathematically expressed by the following equation (1). A waveform of the output voltage V.sub.OUT1 is as shown in FIG. 6. EQU V.sub.A =-Rf.multidot.Vp.multidot..omega..multidot.(C2-C1) (1)
In the capacitor type acceleration sensor thus constructed, when no acceleration acts on the acceleration sensing element 1, the movable electrode 4 is set at the mid position between the fixed electrodes 2 and 3. At this position, the capacitance values of the variable capacitors C1 and the C2 are equal to each other. The amplitude V.sub.A of the output voltage V.sub.OUT1 is zero as seen from the equation (1). The operational amplifier A1 of the capacitance-difference detector circuit 10 produces voltage of zero as a capacitance-difference detect signal. When an acceleration acts on the acceleration sensing element 1 in a direction, the capacitance values of the variable capacitors C1 and C2 vary, so that the amplitude V.sub.A takes a value dependent on the capacitance-difference between the variable capacitors C1 and C2. The output voltage V.sub.OUT1 of the capacitance-difference detector circuit 10 also vary dependent on the capacitance-difference between the variable capacitors C1 and C2. When C1&gt;C2, the output voltage V.sub.OUT1 varies along a waveform as indicated by a solid line in FIG. 6. When C1&lt;C2, viz., an acceleration acts on the acceleration sensing element 1 in the direction that is opposite to that of the electrostatic power in the previous case, the capacitance-difference detect voltage V.sub.OUT1 varies along a waveform as indicated by a dotted line. In this case, it is phased shifted 180.degree. from the output voltage in the previous case.
The direction of the acceleration can be judged by synchronizing the sinusoidal wave signal from the oscillator 6 with the capacitance-difference detect voltage V.sub.OUT1 of the capacitance-difference detector circuit 10 by the synchronous detector circuit 12. When the output voltage of the capacitance-difference detector circuit 10 is applied to the electrostatic power generation/feedback means 15, the electrostatic power generation/feedback means 15 produces the servo feedback voltage that depends on the output voltage of the capacitance-difference detector circuit 10. This servo feedback voltage is outputted as the output voltage of the acceleration sensor. At the same time, an electrostatic power to reduce the capacitance of the variable capacitor C1 is applied to the fixed- and movable-electrode pairs 2 and 4, and 3 and 4, when C1&gt;C2. When C1&lt;C2, an electrostatic power of which the direction is opposite to that of the electrostatic power applied when C1&gt;C2 is applied to the fixed- and movable-electrode pairs 2 and 4, and 3 and 4. In this way, the movable electrode 4 is set at the reference position (neutral position).
To set the offset of the capacitance-difference detect signal at 0 (zero) when the acceleration is 0, it is necessary to adjust the variable capacitors C1 and C2 so as to have exactly equal capacitance values under the condition that the acceleration is 0. Actually, a product variation is inevitable for the manufactured acceleration sensing elements 1. This makes it difficult to adjust capacitance values of the variable capacitors C1 and C2 so as to be exactly equal to each other. As shown in FIG. 6, the phase of the capacitance-difference detect signal is inverted depending on the direction of the acceleration. Because of this, a small capacitance detecting region is within the phase inverted region. As a result, the linearity of the capacitance-difference detect signal is poor in the vicinity of the 0 point, and the resolution of the capacitor type acceleration sensor is low.
The capacitance-difference detect voltage whose phase is inverted depending on the direction of the acceleration, varies with time. In this case, the variation of this voltage takes a sinusoidal waveform varying with respect to the zero voltage, irrespective of the direction of the acceleration. In other words, the capacitance-difference detect voltage is an AC voltage of a sinusoidal waveform. To detect the direction of the acceleration, a synchronous detector circuit is required. Use of the synchronous detector circuit makes the circuit construction complicated, and increases the sensor cost.